In a conventional construction machine, especially a conventional hydraulic excavator or the like, the specifications of a cooling system including a heat exchanger as a cooler for working fluid are optimized so as to ensure a heat balance of a prime mover, a hydraulic system, etc. on the basis of standard operations using a bucket. In this case, in the states of operations performed under severer conditions than in standard operations, such as, for example, a continuous high-load operation, an operation performed in a place of a very high ambient temperature such as in a tunnel, or an operation performed with the construction machine deteriorated, the heat balance is lost, which increases the temperature of the hydraulic system and adversely affects the machine lives of hydraulic devices.
However, if the specifications of the cooling system are optimized beforehand so as to ensure a heat balance under severer conditions than in standard operations such as, for example, a continuous high-load operation, not only the problem of overengineering occurs relative to the standard operations most frequent in general use, but also it is uneconomical. If the capacity of the heat exchanger is increased as a countermeasure, the entire cooling system becomes larger in size, leading to an increase of cost and an increase in size of the construction machine concerned, or the problem may arise that the noise level becomes higher because the cooling air volume needs to be increased.
In connection with such problems, JP-A-2000-110560 discloses a technique wherein the number of revolutions of a cooling fan is controlled in a variable manner to suppress noise during standard operations, and the heat discharge amount of a cooler is increased when the operation is performed in a severer condition than in standard operations.
According to a technique disclosed in Japanese Utility Model Registration No. 2565113, in a working machine wherein when a control lever is in a neutral state (unoperated state), a cooling fan is rotated by a manual operation by an operator and working fluid is cooled by a cooler, the capacity of a variable displacement type hydraulic pump is maximized upon detection of the neutral state of the control lever and the manual operation by the operator, thereby increasing the flow rate of the working fluid passing through the cooler so as to maximize the heat discharge amount of the cooler.    Patent Literature 1: JP, A 2000-110560    Patent Literature 2: Japanese Utility Model Registration No. 2565113